Testing by Subcontractors

Subcontractors supplying mechanical parts to medical device OEMS have never had it easy. There are the ever-increasing tolerance and price issues, and more and more the OEMs are requiring electrical test sequences to ensure meeting of safety parameters. These sequences usually have a basis in published Medical Standards from the IEC, AAMI, or other standards bodies, and most times simplified, greatly, to allow fast, economical testing in a production environment to ensure that the production will be in accordance with particular standard requirements.

We have seen that in some cases, the subcontractor may not have deep expertise in the standards to which they are testing to, in methods by which the test requires conducting, or the pass/fail criteria of the sequence. These subcontractors may be fabricators, assembly locations, or even limited production custom design houses, but the problem remains the same – how to conduct the test sequence, which in some cases is simply a specification of the result requirement?

Any of these scenarios may require the subcontractor to obtain specialized equipment. How does the subcontractor validate the spec, make sure it is applicable and properly specify the needed test equipment to ensure they are testing correctly?

An Electrical Test Requirement Checklist

The equipment specification
The OEM has many different resources at his disposal, which are usable to help him create a test program for a subcontractor. Not only are there different standards and standards issuing bodies, but also, within the standards there tend to be at least two different ways defining tests: Type Testing and Production Testing.

A. Type Testing: This test sequence applies once in the product’s life to make sure the design meets the requirements in the standard the OEM has selected. These tests are much more stringent in nature, and are not, in theory, conducted on production. However, more and more OEMs are requiring type-style tests be applied to 100% of production for some safety-specific components of the design. When the OEM on your production contemplates a type-testing program, you might think about the following points.

1. For the type of equipment being tested, is the standard the correct one? You can make this determination easily, and expertise in the standard is not required. Google the name or reference number of the standard, and find the reference to it on the issuing body’s website. In other words, if it is an IEC Standard, click the link on the iec.com result. Then, you can read the name and scope of the Standard free. To test the product you are building, do you think this is the correct standard? Most cases will be yes, but it never hurts to check. If the name of the standard does not correlate, it is certainly worth a phone call to your OEM to make sure that the test specification is correct for your product.

1. Many newer production line tests use an exponential decay waveform like the one pictured. They are a quick and safe way to evaluate insulation systems used in motors, circuit components, and end use products. 2. Bi-phasic wave form, as required by AMMI DF-1 3. Current wave form requirement from EN61000-4-5

2. Does the specification received for the type test agree with the version of the test in the latest copy of the Standard? The latest version is available online; if the standard reference of the test you are to perform is different, there could be a good reason but clarification from the OEM or internal test specifier would be prudent. This is extremely important; if the year of issue or the version number of the standard you are being asked to reference by your OEM is not the same as the online version, the actually referee document needs to be defined at the outset.

3. Does the specification received include the standard paragraph, including the requirements for all tables and footnotes, and is the specification in accordance with the standard? Even if the OEM test is adapted from a standard’s type test, it makes sense to check the entire spec to make sure something is not missed. A footnote can change or possibly waive the test. The information presented below can be tough going if you do not have expertise with the referenced standard, but we are including this information to be included in the discussion with your OEM, to make sure the test being required is correct and meaningful.

a. Is the test performed while the equipment is energized? Conduction of most production tests is without power applied, but type tests can specify this condition. It is a good thing to consult the OEM or internal test specifier if this is not defined.

b. Is the test voltage correct for the iInsulation class? In cases where the surge test is checking for a dielectric breakdown, a table showing the insulation class of the tested part, and the voltage potential it sees during operation usually assigns output voltage. If so, consult with the OEM or internal test specifier about which Insulation Class he/she is testing to, and the voltage level, to double check the proper test voltage. If this voltage is set too high, the test may overstress the insulation.

c. Is the test voltage correct for the creepage and clearance distances? Sometimes used are surge tests, to judge the effectiveness of air gap insulation. The clearance distances, and the surge voltage for that clearance, are in tables in the standard. These should be double-checked.

B. Production Testing – does it test your product?

1. Is this test reasonable in scope? Many production tests consist of a dielectric withstand and ground continuity test. These tests concern only the primary circuits of the part. If your part does not see primary voltage, these tests may not be required.

2. Does the test have a reasonable pass/fail point? If you are asked to perform a hipot test between mains and ground, it should be in the range of 1,500VAC/2,100VDC. If the test is between primary and secondary, it should be in the range of 2,500VAC and 3,000VAC. If your test spec exceeds these limits by a considerable margin, it is time to confirm with the OEM.

The test specification

The specificiation, whether derived from a type test or a production line test specification, must adequately test the part and make sense from a production standpoint. The following is a checklist that may be helpful when the OEM gives you a test package.

A. Does it make sense? As a first check, the subcontractor should evaluate the specification for applicability to the parts going to the OEM, and check to make sure the test required has a result that will evaluate the specification. In addition, the subcontractor should evaluate the time and materials required to conduct the test, so time and equipment costs can be included in the build.

Top: These surge testers generate exponential decay waveforms. Depending on the type of equipment, the peak voltage and times of the waveform vary. Bottom: Hipot verification tester.

B. Does it test the parameter in question? It is extremely important to take a close look at the test specification, including the point in the circuit or on the part, where the test is applied. Does it physically test your part?

C. How will the test impact production? What is the overall impact the test will make on your production? How long will the test take to conduct, and will this time negatively affect the production schedule? Should there be consideration for more than one tester to keep throughput acceptable?

D. How will your facility handle the testing? Does this area of the plant have infrastructure to support the test (personnel, power, network connections, etc.)? Is it designable to keep test personnel safe, and other personnel clear of the danger if the test uses high voltage or high current?

E. Is the test specification complete regarding pass/fail criteria and disposition of failing material? What exactly comprises a failure? Can rework and repeat testing occur to the failing part, or is it to be disposed of?

F. Did you schedule check testing early enough? Make sure the design engineers know about the test up front, and that qualification tests scheduled early enough so any test failures have time for evaluation in order to deal with them in an economical fashion.

Other considerations

A. Is the test accurate enough to find a sample that is prone to failure in the field? This point is difficult to determine. Make sure the test makes sense for the product requiring testing. For example, if your product has main voltage connections, it is not appropriate to apply the 500V hipot test for secondary parts. Testing should be appropriate, so potential field failures have a chance of being found, and fixed in the manufacturing facility.

B. Is the throughput acceptable? Timing of tests should be to make sure that there is enough parallel capacity to keep production on schedule. If a test is too slow, multiplexing outputs or duplicating test stations can keep schedules intact.

C. Is the method of test control specified? Are you to keep records of all failed parts, or of all tested parts? Is this record keeping manual or by the test equipment? Is the test equipment to stand alone, or run with the assistance of a computer or PLC? What are the portability requirements; does it require design to move, easily, to another location or is it for a fixed place?

D. Is the testing being done safely? Most importantly, it is crucial to ensure that design of the equipment is to keep operators and uninvolved plant personnel safe.

While OEMs are placing new demands on subcontractors, a simple evaluation of a test plan can help formulate your implementation plan. Of primary importance is to determine if application of the test plan is proper to your product and if the required results make sense. Communication with the OEM is extremely important to make sure there is correct understanding of the plan, for implementation to the satisfaction of both parties, prior execution of the purchase order.

Compliance West USA
San Diego, CA
compwest.com